The effects of NaOH pretreatment temperature on the physicochemical characteristics and methane production of anaerobically digested banana stems were investigated in this paper. With the increase of pretreatment temperature from 0 °C to 100 °C, the chemical oxygen demand (COD) of the soak liquid in the treated biomass approximately linearly increased from 5.9 g/L for the untreated stems to 34.0 g/L. A weight loss of 5.1% was observed for the untreated material, while it was up to 31.2% for the sample treated at 100 °C. The removal of lignin and hemicellulose accounted for the majority of the weight loss. The removal rates of lignin and hemicellulose increased from 15.0% to 41.6% and 1.9% to 23.6% when the treatment temperature increased from 0 °C to 100 °C, respectively. Moreover, the crystalline index (CI) of the banana stems also increased with rising temperature, resulting from the dissolution of amorphous cellulose with increasingly harsher alkaline environments. The optimal pretreatment temperature for banana stems was confirmed at 50 °C. In these conditions, methane was produced via anaerobic digestion with 239.9 mL/g total solid (TS) yield, which represented an increase of 66.7% over untreated banana stems.